10

I am searching for an explanation what exactly the output of the commands ip link and ip addr means on a linux box.

# ip link
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
link/ether 00:a1:ba:51:4c:11 brd ff:ff:ff:ff:ff:ff
4: eth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT qlen 1000
link/ether 00:a1:ba:51:4c:12 brd ff:ff:ff:ff:ff:ff

What exactly are the LOWER_UP, NO-CARRIER and other flags? I have found a reference at http://swlx01.hs-esslingen.de/doc/usr-doc/iproute-2.6.22/ip-cref.ps but it does not contain complete information and man pages are not detailed enough.

14

Those are interface's flags. They are documented in the netdevice(7) man-page. Below is the relevant part (reordered alphabetically):

IFF_ALLMULTI      Receive all multicast packets.
IFF_AUTOMEDIA     Auto media selection active.
IFF_BROADCAST     Valid broadcast address set.
IFF_DEBUG         Internal debugging flag.
IFF_DORMANT       Driver signals dormant (since Linux 2.6.17)
IFF_DYNAMIC       The addresses are lost when the interface goes down.
IFF_ECHO          Echo sent packets (since Linux 2.6.25)
IFF_LOOPBACK      Interface is a loopback interface.
IFF_LOWER_UP      Driver signals L1 up (since Linux 2.6.17)
IFF_MASTER        Master of a load balancing bundle.
IFF_MULTICAST     Supports multicast
IFF_NOARP         No arp protocol, L2 destination address not set.
IFF_NOTRAILERS    Avoid use of trailers.
IFF_POINTOPOINT   Interface is a point-to-point link.
IFF_PORTSEL       Is able to select media type via ifmap.
IFF_PROMISC       Interface is in promiscuous mode. 
IFF_RUNNING       Resources allocated.
IFF_SLAVE         Slave of a load balancing bundle.
IFF_UP            Interface is running.

So, LOWER_UP means there is a signal at the physical level (i.e. something active is plugged in the network interface). NO-CARRIER, is the exact opposite: no signal is detected at the physical level.

  • 1
    Interesting how NO-CARRIER is not documented in netdevice(7)... – Peter V. Mørch Nov 20 '18 at 14:28
  • This is because NO-CARRIER is not an actual flag that you can get or set. It is defined as a combination of IFF_UP and not IFF_RUNNING. – xhienne Dec 28 '18 at 17:06

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